Electromagnetic components such as inductors and transformers have traditionally been constructed by winding one or more conductors about a cylindrical or torroidal core. This method of construction requires that a conductor, such as a wire, be wrapped around the outer surface of the core. The resulting components are expensive and time consuming to manufacture, and do not readily lend themselves to miniaturization or automated assembly.
More recently, electromagnetic components have been constructed using printed circuit board (PCB) manufacturing techniques, where windings and individual winding turns are formed from one or more conducting layers patterned on the surface of an insulating PCB layer, or on one or more layers of a multilayer PCB. The use of PCB conductive traces as windings has several advantages over conventional, wound windings. First, the assembled PCB winding has a smaller mounting footprint than a conventional winding, since it does not need extra leads or soldering pads. Second, the PCB winding assembly is much simpler than conventional windings, since the winding and other components in the winding circuit of a multilayer PCB can be board mounted using the same reflow and automation processes used to mount other components. Third, a multi-layer PCB winding has improved reliability since the likelihood of shorting across adjacent turns of the winding is greatly reduced or substantially eliminated. It is a well known problem of prior art power chokes formed using layers of stacked metal foils separated by insulators that shorting between layers is much more likely to occur.
In a multi-layer PCB, a PCB winding is formed from a plurality of patterned conductive traces, typically of copper, each formed on a separate insulating layer of the multi-layer PCB. Each trace forms a nearly closed typically circular pattern, so as to create the electromagnetic equivalent of one turn or loop of a prior art wire formed winding. Terminal points are formed at the ends of each trace for making connections to other traces, so as to form the individual turns of the winding. For example, the pattern can be a “C” shape with a terminal point at each of the two extreme points of the C. The PCB winding is formed by connecting the traces from different layers of the PCB through the intervening insulating PCB layers. These connections are typically plated through holes or vias in the PCB insulating layers. The traces can be connected in various ways. The traces can all be connected in series to form a winding where each trace is a separate turn of the winding. In this example, the terminal ends of each trace are offset from the traces on the adjacent levels, so that the plated through holes in each level do not intersect. Two or more traces can also be connected in parallel to decrease the impedance of a particular turn of the winding. The resultant winding is a function of the way in which the conductive traces on each layer of the multi-layer PCB are connected together and coupled to external circuits.
The inductance of a winding formed using a multi-layer PCB can be increased by introducing a core of a magnetic material through an aperture formed in the PCB layers that extends through a central non-conducting region of each layer. The core is typically included as part of a housing for the multi-layer PCB winding.
Conductive leads or vias are included on one or more layers of the multi-layer PCB to enable the efficient electrical connection of such components to an external circuit, for example by surface mounting and reflow soldering of the component to other components mounted on the same PCB or to another PCB having such other circuit components. This use of a multi-layer PCB to fabricate electromagnetic components results in smaller, more easily manufactured, and more reproducible components than is possible using a winding formed from a wire wrapped about a core.
Windings constructed from two or more conducting layers of a multi-layer PCB have many advantages over conventional wire windings, but have problems that result from the structure of PCBs. One problem with multi-layer PCB windings results from their having thin conducting layers separated by insulating material. The high current carrying capacity required for some types of inductors, such as power chokes, can result in excessive heating and thus a reduced lifetime for the component. Current carrying capacity of the winding can be increased by increasing the number of PCB layers in the multi-layer PCB and connecting the conductive traces on these new layers in parallel with pre-existing conductive layers on other layers of the PCB, but this is an expensive option since the cost of an inductor formed in a multi-layer PCB is proportional to the number of layers and the weight of the copper used in each layer. To handle a high current of over 40 amps with a two or three turn winding with low loss, a PCB having eight to ten layers will require approximately 4 ounces of copper.
What is needed is an improved winding for an inductor that is formed from a multi-layer PCB and that allows for higher current flow without a corresponding increase in temperature, or alternatively allows for fewer layers in the PCB, and which provides increased manufacturing and layout efficiencies. The resulting device should be compatible with PCB surface mounting manufacturing techniques and should be less expensive than prior art devices whose windings are formed solely from multi-layer PCBs.